Insulation board

What is insulation board?

Insulation board is a rigid panel used to reduce heat transfer through building assemblies. Unlike fluffy batts, these sheets create a flat, continuous layer that limits thermal bridging at studs and joists. They are made from materials such as foam plastics, mineral fibers, or natural cork, and may include facings like foil or plastic film to improve moisture and air control. Boards are lightweight, easy to cut, and can be installed with screws, cap nails, or construction adhesive.

Where you might use it

Homeowners and pros use insulation board in many spots:

• Exterior walls as continuous insulation over sheathing
• Interior basement and crawlspace walls
• Roof decks and attic kneewalls
• Under slab-on-grade floors or over concrete slabs
• Rim joists and band joists
• Garage doors and attic hatch covers
• Sound control behind drywall or under floating floors (mineral wool or cork)

Using rigid boards as a continuous layer improves comfort and lowers energy bills by reducing drafts and cold spots.

Types of insulation board

Different boards suit different jobs. Here are the common options and where they shine.

• EPS (expanded polystyrene): R about 3.6 to 4.2 per inch. Affordable, vapor semi-permeable, available in many densities. Works for below-grade with proper drainage. Used on walls, roofs, and under slabs.

• XPS (extruded polystyrene): R about 4.7 to 5 per inch. Higher initial moisture resistance and compressive strength than standard EPS. Often used on foundations, slabs, and as exterior wall insulation. Choose low global-warming-potential products where available.

• Polyiso (polyisocyanurate): R about 5.6 to 6.5 per inch at moderate temperatures. Often foil-faced and excellent for above-grade walls and roofs. Performance can drop in very cold conditions; not recommended in direct soil contact.

• Mineral wool board (stone wool): R about 4.0 to 4.3 per inch. Noncombustible, vapor open, drains water, and provides sound control. Good for exterior rainscreen walls and interior acoustic upgrades; not for direct burial or constant wetting.

• Cork board: R about 3.6 per inch. Natural, renewable, good acoustic performance. Often used as interior linings or underlayment rather than structural exterior insulation.

• Phenolic foam boards: Higher fire resistance than many foams, R around 4.5 to 4.8 per inch. Less common in residential stores but used in some roof systems.

Facings and coatings

• Foil-faced boards: Reflect radiant heat when they face an air space. Provide a strong vapor barrier and are easy to tape for air sealing.

• Fiber-reinforced or film-faced boards: Add durability, help with air control, and make taping seams easier.

How to choose the right board

Consider these factors before buying:

• Location and exposure: Below grade or in contact with concrete favors EPS or XPS. Above-grade walls and roofs often use polyiso or mineral wool.

• R-value target: Check your local energy code and climate zone. Thicker boards raise R-value. Stacking two layers with staggered seams improves performance.

• Moisture behavior: If you need drying potential, favor vapor-open materials such as EPS or mineral wool. If you need a strong vapor retarder, foil-faced polyiso can provide it.

• Fire and ignition: Foam plastics typically must be covered by a code-approved thermal barrier on the interior, such as half-inch gypsum board. In garages and basements, confirm whether an ignition barrier or full thermal barrier is required.

• Compressive strength: Under slabs or under siding furring, choose boards rated for the load. Look for the PSI rating on the product data sheet.

• Compatibility: Use adhesives and tapes approved by the manufacturer. Solvent-based products can damage foam.

• Pests: In termite-heavy regions, consider termite-resistant foam, inspection gaps at foundations, or metal flashing details.

Installation tips for solid results

• Plan the layers: If possible, use two thinner layers rather than one thick layer, staggering the seams to reduce air leakage paths.

• Cut cleanly: Score and snap foam with a utility knife and straightedge, or use a fine-tooth hand saw. Cut mineral wool board with a serrated insulation knife.

• Fasten securely: Use cap nails or screws with large washers on framed walls. Over exterior sheathing, attach boards and then add vertical furring strips through to studs for siding. On concrete, use foam-safe adhesive plus masonry fasteners as needed.

• Seal the seams: Tape seams with compatible tape. Use foil tape on foil-faced boards and high-quality acrylic sheathing tape on others. Seal penetrations with one-part spray foam or sealant.

• Manage water: On exteriors, integrate flashing and weather-resistive barrier details so water drains to the outside. Mineral wool boards can sit outboard of a housewrap as part of a ventilated rainscreen.

• Mind the gaps: Fit boards snugly at edges and around windows. Small gaps can be sealed with spray foam. Larger voids should be recut for a tight fit.

• Protect from UV: Foam boards degrade in sunlight. Cover promptly with cladding, roofing, or drywall.

• Meet fire codes: Do not leave foam board exposed inside living spaces unless the product rating and jurisdiction allow it. Most situations require a gypsum board cover.

Maintenance

Insulation board is low-maintenance once enclosed. Periodically check accessible areas such as basements and rim joists for:

• Moisture or condensation at seams
• Pest activity along edges
• Tape adhesion and damage from impacts

Repair lifted tape and reseal gaps as needed. Exterior boards should remain protected by cladding and flashing.

Common mistakes to avoid

• Using the wrong board below grade: Polyiso and mineral wool are not suited for direct soil contact. Choose EPS or XPS with proper drainage.

• Skipping seam sealing: Unsealed seams allow air leaks that reduce performance.

• Ignoring code-required fire protection: Foam plastics usually need a thermal or ignition barrier on the interior side.

• Solvent adhesives: Products with strong solvents can melt foam. Use foam-safe adhesives and tapes.

• No drainage plane: When adding continuous insulation outside, integrate flashing and water management. Do not trap water behind cladding.

• Exposed foam in sunlight: Cover promptly to prevent degradation.

• Compressing boards: Do not overtighten fasteners or crush boards under furring beyond their load rating.

Related terms

• R-value: A measure of thermal resistance; higher is better.
• Continuous insulation (ci): Uninterrupted insulation layer that covers framing to reduce thermal bridges.
• Thermal bridge: A conductive path, like a stud, that bypasses insulation.
• Vapor retarder and vapor barrier: Materials that slow or block vapor diffusion.
• Housewrap and WRB: Weather-resistive barriers that manage liquid water and air.
• Spray foam and batt insulation: Alternative insulation types that can pair with rigid boards.

Real-world examples

• Basement wall upgrade: Adhere 1.5 to 2 inches of EPS or XPS to concrete with foam-safe adhesive, tape seams, seal the rim joist, then frame a 2x4 wall in front and finish with drywall to meet fire requirements.

• Exterior wall retrofit: Over existing sheathing, install two layers of 1-inch mineral wool or polyiso with staggered seams, tape as appropriate, add vertical furring strips through to studs, and reinstall siding. This reduces thermal bridging and boosts comfort.

• Attic hatch cover: Build a lightweight box from 2-inch foil-faced polyiso, tape all seams, and add weatherstripping at the hatch perimeter to cut drafts.

• Garage door panels: Cut thin foam boards to fit between door rails. Use foam-safe adhesive and light clips. Be mindful that added weight can affect door balance; adjust springs if needed by a qualified technician.

• Under-slab insulation: Before pouring a slab, place 2 inches of Type II or higher EPS or XPS over compacted base, tape seams, and protect with a vapor barrier as specified by code.